The invention relates to a television special effects generator comprising: an input for receiving a video signal in the form of successive samples encoded in digital form and corresponding to picture elements on display, the display being built up from line and field sequential scans; a frame store for storing video signals forming a television frame; input addressing means for writing the input video signal samples into desired locations in the store; output addressing means for reading video signal samples from selected locations in the store; and an output coupled to the store for receiving successive video signal samples read from the store as the output video signal; in which the output addressing means is capable of generating read address signals which cause the store locations to be read in a sequence which corresponds to incrementally increasing (or decreasing) picture lines in a television field until a given point is reached and then incrementally decreasing (or increasing) picture lines; characterized in that for alternate television fields the stepping of the output addresses is inhibited for one line period when the given point is reached.
Such special effects generators are normally capable of expanding (zooming) and compressing (squeezing) a television picture and of moving the picture to selected parts of the display screen. They may also be capable of causing multiple pictures to be displayed and of rotating a picture about an axis perpendicular to the plane of the picture and one or more axes in the plane of the picture. Such generators have also been used for displaying a reversed picture which, if a squeeze-zoom action is combined with a picture direction reversal, can produce an effect in which the picture appears to "flip" like a card being turned over. These effects are produced by appropriate control of the write and read addresses of the frame store which are normally controlled by means of counters. A reflection effect has been produced by mixing a picture with a reversed picture and ensuring that the center line matches. This gives a mirror image of the left hand side of the screen in the right hand side. This effect is, however, complicated to set up and requires a two channel mixer for its achievement.
U.K. Pat. No. 1583928 discloses a television effects generator in which the output addressing means comprises up/down counters whose counting directions are reversed to produce a mirror effect. This provides a simple means for producing a mirror image of one half of the displayed picture in the other half of the displayed picture. All that is required is a simple instruction to the read address generator to reverse its sequence of addresses when a given point is reached to cause samples from the store to be read out in reverse order. Thus the need for two channels to produce this effect is eliminated.
However, a straight reversal of the addresses produced by the address generator causes a jagged edge to be produced on diagonal lines. This could apparently be overcome by causing the fields to be read in reverse order but this causes one field to be read in one part of the picture and the other field in the other part of the picture. This is not noticeable, if there is no motion in the picture, but if there is motion in the picture it manifests itself as movement in one part lagging that in the other part and is particularly noticeable at the boundary between the two parts.